The parallelist argumentation is still used today. There cannot be more than one object at the same time in the same space; but there can be several thoughts at the same time in the same space (in an individual). I cannot extend both arms, as on a cross, and touch my chest at the same time; but I can have the desire to do both gestures at the same time. The parallelists of the seventeenth century think that matter acts on matter and reacts only to matter. In similar fashion, the mind can only act on the mind, not on matter. According to Husserl, such a clear distinction between the substance of the mind and that of matter is a creation of the seventeenth century. In the preceding centuries, the differences between body and soul were more fluid.
A physical event is necessarily caused by an event that precedes it, whereas two thoughts that follow one another are not necessarily related and can be part of two independent causal systems. The soul and the body cannot interact directly because their functioning is so different. They are therefore part of two distinct dimensions.
Descartes had, at one moment, put forth the idea that the soul is the assembly of the organs,â7 that is, the organization of the organs that constitutes an organism But he did not have the means to explore this hypothesis. It is nonetheless so tempting that the subsequent generation of philosophers like Spinoza found ways to tease something out of this analysis.
This argumentation is different from that of Aristotle, for whom affects and logical thoughts emerge in a soul that is a dimension of the individual system: